Technically speaking, yes (especially if you have a tracking antenna array) but realistically, you will need a bit more. I have been successful with 5 watts and a whip antenna on my D7 but even that takes some good timing. Both satellites see a really big part of the world and you will be trying to get in while many others are too.

I wonder if we could have an antenna poll to see what types of antennas people are using?
For example: directional (Yagi, log-periodic, beam), ground-plane, discone, eggbeater, dipole, mag mount, other.

I use an Arrow GP 146/440 ground-plane antenna indoors with 25 watts.

Peter - AF6DS

P.S. PCsat is also now operational again on the same packet simplex frequency as the ISS.

If you have a decent antenna, power is ALMOST pointless. (After making a few adjustments via the FT-817ND's Hidden Menu, I have the lowest power setting adjusted back to 250 mW, I'll be trying the ISS at 1/4 watt today.)